Piston engines for general aviation are typically large displacement, direct drive, air-cooled, spark-ignition engines that burn leaded aviation fuel and typically run between 2300 and 2700 rpm. At higher revolutions per minute (“rpm”), the propeller speed approaches the speed of sound with corresponding decreases in efficiency.
Aviation fuel is considered a boutique fuel; it is expensive and not available in many parts of the world. The future of the fuel is also in question because it contains lead. For these reasons, considerable effort has been expended to develop viable compression ignition engines for general aviation to burn jet and diesel fuel. In addition to burning jet fuel, which is widely available and cheaper, compression ignition engines are more efficient, and jet fuel and diesel fuel have higher energy densities than aviation gasoline. The jet fuel burning turboprop and turbojet solutions with power, weight and reliability advantages over reciprocating engines, are not economically viable for much of general aviation at this time because the costs and fuel burn are much higher.
Engine power is directly related to pressure on the pistons, engine displacement and rpm. In order to generate the necessary power at lower rpm needed for the propeller using direct propeller drive, standard aviation spark ignition engines have large displacements. For example, the Continental TSIO-550-K has a displacement of 9.05 l (550 in3). It generates 315 hp (235 kW) at 2500 rpm and has an uninstalled weight of 578.4 lbs (262.4 kg) with twin turbochargers. It is used in the most commercially successful single engine general aviation aircraft currently available, the Cirrus SR22T, which is a 4/5 place aircraft with 3600 lbs. gross weight and typically flies at cruise speeds approaching 200 kts (230 mph). The TSIO-550 and its TIO-540 Lycoming counterpart spark ignition engine series represent the current state of the art in general aviation and reflects the power and weight targets to be achieved in the preferred embodiment of this invention.
Efforts to develop viable compression ignition engines for general aviation have been met with difficulty. These engines typically weigh more than spark ignition engines to constrain the increases in pressure generated. Current efforts include both larger displacement air and oil cooled direct drive engines and smaller displacement liquid cooled engines running at higher rpm and incorporating propeller speed reduction gearboxes. The larger displacement direct drive engines may not succeed in meeting the weight requirements to produce power in the 300 hp range. The gearboxes of smaller displacement engines are susceptible to the potentially destructive effects of torsional vibration characteristic of compression ignition engines.
Thielert, now owned by Continental Motors (which is a subsidiary of the Aviation Industry Corporation of China (AVIC), in turn owned by the Peoples Republic of China), introduced the first commercially successful application of a compression ignition engine for general aviation, the Centurion 1.7. It is a 1.689 l (103 in3), 134 kg (295 lbs.) turbocharged engine producing 135 hp (101 kW) based on a Mercedes 4-cylinder in line, liquid cooled, double overhead camshaft engine. An aluminum alloy block was substituted for the iron block used in Mercedes engines. The engine was certified by EASA (“European Aviation Safety Agency”) in 2002 and the FAA (“Federal Aviation Administration”) in 2003. Production ceased in 2006 when production of the Centurion 2.0 (now CD 135) began. The 2.0 is a 1.991 liter (121.5 in3) variant, with the same weight and power as the 1.7, which also uses a Mercedes engine block. This was followed later by the 2.0S (now CD 155) with 155 hp (114 kW). The rpm of these engines at maximum power is 3890 with a propeller speed reduction gearbox ratio of 1:1.69.
Problems with these compression ignition engines have been significant. They suffer from a relatively poor reliability, as compared to air-cooled spark ignition aviation piston engines, for which the expected time before overhaul (“TBO”) is 2000 hrs. The early Centurion 1.7 l variant was discontinued in 2006. The time before replacement (“TBR”) for the engine is 1000 hours with mandatory clutch and gearbox teardown and inspection every 300 hours. The Centurion 2.0 has a TBR of 1500 hrs. with gearbox inspections every 600 hrs. The Centurion 2.0S has a TBR of 1200 hours, with gearbox inspections every 600 hours. The development of the V8 Centurion 4, 3.996 l (243.9 in3), 350 hp (246 kW) with gearbox and weighing 600 lbs. (272.2 kg) uninstalled has not been completed. Continental Motors is now developing the CD 300, a V6, 3 l variant that generates 310 hp (228 kW) at 2300 rpm. It is adapted from a Mercedes auto engine with gear reduction and weighs 560 lbs. uninstalled. There may be multiple reasons for the relatively poor reliability of these engines and their gearboxes, but a major factor is likely the torsional vibration inherent in compression ignition engines. These problems may even be greater for the higher power CD 300 with six-cylinder, four-stroke dominant 3rd order harmonic torsional excitation.
Because of dissatisfaction with the Centurion engines, Diamond Aircraft Industries, the initial prime user of the Centurion engines, founded Austro Engines with its partners in 2007 and has developed the AE 300. The AE 300 engine is a 4-cylinder, in line, liquid cooled, turbocharged, 1.991 l engine that generates 168 hp (123.5 kW) at 3880 rpm and weighs 414 lbs. (185 kg) uninstalled. It also is based on a Mercedes engine block and has a speed reduction gearbox (1:1.69). It retains the heavier iron block design of the original Mercedes engine. It has received certification for a TBO of 1800 hours for the engine and gearbox from EASA.
SMA (Societe de Motorisations Aeronautiques), a subsidiary of Snecma (Safran), has developed the SR305 compression ignition engine, which is similar in design to existing general aviation spark ignition engines. The SR305 is an opposed four cylinder, air and oil cooled, direct drive, compression ignition engine with a displacement of 5 l (305 in3), which produces 227 hp (169.3 kW) at 2200 rpm and weighs 455 lbs. (206.4 kg) uninstalled. It has a TBO of 2400 hrs. It was certified by EASA and the FAA in 2011. SMA is working on a 260-280 hp variant and is also developing the SR460, a 6-cylinder 300 hp range version, which likely will weigh more than 600 lbs. uninstalled and may not meet the weight requirements for the Cirrus SR22 class of aircraft. Continental Motors has purchased the rights to the smaller SR305. Its own version, the CD 230 has been certified and they are working on a CD 250 variant
Engineered Propulsion Systems (“EPS”) is developing an aero diesel engine. Its approach has been to use a 180 degree V8 liquid cooled, turbocharged engine with overhead camshafts and 4.4 l displacement to achieve 350 hp with gear reduction. EPS is proposing to reduce engine vibration by arranging the cylinders horizontally opposed with paired throw crankpins, using a compacted graphite iron crankcase, a first order balance shaft, an internal torsional vibration absorber and tuned isolation. It is uncertain whether this engine will escape the issues that have plagued the previous compression ignition engines with gearboxes. The uninstalled weight is about 600 lbs. and when installed may not meet the weight requirements for the Cirrus SR22 class of general aviation aircraft.
Previous attempts have been made to avoid the need for a separate propeller speed reduction gearbox by proposing pushrod engine configurations that drive the propeller in concert with the camshaft. In a four stoke internal combustion engine, a camshaft is used to mechanically control the opening of the intake and exhaust valves during the engine cycle to allow the charging of the cylinders with air or air fuel mixtures and to exhaust the burnt gases. Camshafts, by nature of their function in a four stoke internal combustion piston engine, rotate at ½ the rate as the crankshaft. In configurations where the camshaft is not placed overhead of the crankshaft, it opens and closes the intake and exhaust valves by driving pushrods acting on rocker arms. The camshaft is commonly driven by gears or chains connected with the crankshaft. Camshafts are also commonly placed overhead of the valves either singly or in pairs where they drive the intake and exhaust valves directly and in this instance would be less suitable as power output shafts.
Engine configurations that drive the propeller with a shaft in common with the camshaft are described in U.S. Pat. No. 1,438,289 to Barbarou, U.S. Pat. No. 1,580,082 to Ybarra, U.S. Pat. No. 1,627,108 to Morehouse, U.S. Pat. No. 3,421,490 to Wiseman and U.S. Pat. No. 3,447,515 to Wiseman, et al. All of these patents disclose a shaft in common with the camshaft driving the propeller with the gearing at the front of the engine close to the propeller. As explained in this specification, however, engines that utilize a camshaft for power output to the propeller with forward gear reduction (sometimes referred to herein as the CDSE-FG configuration) will still suffer from significant torsional vibration similar to the standard gear reduction configurations where a dedicated gearbox is mounted at the front of the engine and connected to the propeller with a short propeller drive shaft with corresponding high maintenance and reliability problems.
U.S. Pat. No. 5,513,601 to Benson discloses a V-8 spark ignition engine with a dedicated propeller shaft and separate camshafts. The propeller shaft is coupled to the crankshaft via rear gearing and the camshafts are coupled to the propeller shaft via timing belts. According to Benson, the longer propeller shaft of this arrangement allows for more solid bearing support to absorb torsional and other loads originating from the propeller, which Benson claims reduces the transfer of these forces to the gears, crankshaft and other engine parts. Benson fails to consider the use of the camshaft with rear gearing to drive the propeller.
There is a need, therefore, for an internal combustion piston engine for aircraft that is suitable for burning jet or diesel fuel, which meets the weight requirements for the Cirrus SR22 class of general aviation aircraft, and which is more reliable and requires less maintenance than previous engines. It is an object of the present invention to provide such an apparatus. Such an engine might also find use in other applications including unmanned aircraft, marine craft and certain land vehicles.
Additional objects and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.